Field of the Invention
The present invention relates to a radar apparatus which radiates laser light toward an object, detects the laser light reflected from the object, and measures or determines the sign of velocity of the object.
The term “velocity” used herein is scalar and refers to the magnitude of an orthogonal projection vector obtained by projecting a relative velocity vector of an object to the radar apparatus onto the optical axis of laser light. Therefore, the direction in which the object is present can be determined by the direction of a laser beam, and the magnitude of the orthogonal projection vector is the velocity.
Background Art
There has been conventionally known a radar apparatus (LIDAR) which emits laser light toward an object, and receives and analyzes the laser light reflected from the object, to thereby measure the distance and velocity of the object.
Use of such a radar apparatus in vehicles has been promoted. For example, a radar apparatus is mounted on a vehicle so as to detect the distance and velocity of an object such as a pedestrian or a vehicle travelling in front of the radar-mounted vehicle, to thereby avoid collision with the object. In such a case, the sign of velocity (whether the object is approaching the vehicle or is moving away from the vehicle) is very important information.
According to one method, velocity is obtained through measurement of a Doppler frequency fd. In such a case, the sign of the Doppler frequency fd must be determined in order to determine the sign of velocity. Conventionally, laser light is FSK modulated through use of AOM (Acoust-Optic Modulator) in order to determine the sign of Doppler frequency fd.
For example, in Patent Document 1, the sign of the Doppler frequency fd is determined as follows. Laser light having a frequency f0 is FSK modulated so as to modulate the frequency to f0+f1 and f0+f2. The frequency-modulated laser light is radiated toward an object, and reflection light from the object is received. The received laser light is mixed with local light with the frequency f0 for optical heterodyne detection so as to convert the received light to an electric signal whose frequency assumes two values, fd+f1 and fd+f2. Subsequently, the electric signal is multiplied by a signal whose frequency is (f1+f2)/2 so as to convert the electric signal to a signal whose frequency assumes two values, (f2−f1)/2−fd and (f2−f1)/2+fd. Further, the resultant respective signals undergo quadrature detection with a signal whose frequency is (f2−f1)/2, whereby an I component and Q component are obtained, each of which has frequencies fd and −fd. Subsequently, FFT (fast Fourier transform) is performed on the IQ component signal which is equal to I(t)+jQ(t) so as to calculate its frequency spectrum and obtain the Doppler frequency fd. Since quadrature detection is performed, the sign of the Doppler frequency fd can be determined, whereby the sign of velocity can be determined from the sign of the Doppler frequency fd.
Patent Document 1: Japanese Patent No. 3872082
Hover, since AOM is expensive, there has been a problem in that such a radar apparatus entails high cost. Also, since AOM utilizes surface acoustic waves, its integration is difficult. Therefore, reducing the cost and size of a radar apparatus has been difficult.